Use of liquid smoke solutions as a replacement for smoking by direct contact with smoke produced from wood has become a standard industry practice. When applied to the surface of meats and other proteinaceous foodstuffs, liquid smoke will not only give the item a characteristic smoke flavor, but will react with the proteins to produce the dark color typical of smoked foods.
One such liquid smoke preparation used commercially, for surface applications is the aqueous smoke flavoring described in Hollenbeck U.S. Pat. No. 3,106,473. This product is produced by partial combustion of hardwood sawdust with limited access to air, followed by subsequent solvation of the desirable smoke constituents into water. A heavy, water insoluble phase which contains tar, polymers, polycyclic aromatic hydrocarbons including benzo[a]pyrene, waxes and other undesirable products unsuitable for use in food applications is discarded.
Smoke is a complex and variable mixture of chemicals which are produced from pyrolysis reactions and includes vaporous compounds which are normally liquid at room temperature. Pyrolysis is a general term for the thermal decomposition of any organic material (i.e. wood, plants, fossil fuels etc.) and can occur during a combustion process or in the absence of combustion. In the former, the oxidation or burning of a portion of the organic matter provides the heat required to vaporize and decompose the remainder. In the absence of combustion, heat must be supplied indirectly from some other source (i.e. radiation, a solid or gaseous heat carrier, or conduction through reactor walls, etc.).
Pyrolysis produces liquids (i.e. condensable vapors), gases (non-condensables) and solids (char and ash) in varying proportions depending upon reaction conditions. The liquids can be further sub-divided into water soluble organics and non-water soluble tars. It is known that the desirable active ingredients for smoke flavoring are among the water soluble condensable vapors (liquids).
Currently liquid smoke is made using conventional pyrolysis which is characterized by relatively slow thermal reactions occurring at moderate temperatures. In the commercial processes, the wood feedstock is dried and ground to sawdust and fed to a reactor system. A typical average reactor temperature is approximately 420.degree. C. Depending on the method of heating, the temperature gradient in the reactor may be from 600.degree. C. at the heater to 250.degree. C. at the bulk wood surface. Residence times of solids (wood/char) and vapors are approximately 10 minutes and 1 minute respectively.
Conventional pyrolysis produces liquid, gas and char yields which are typically 35, 35 and 30% by mass of the wood feedstock, respectively. Since the water insoluble constituents are between 50 and 65% of the total liquids derived from the wood content, the net yield of raw liquid smoke is relatively low (i.e. 12 to 20% of the wood feedstock).
The pyrolysis products are often passed through a water bath or scrubber. The gaseous products pass through the water bath. The solids and water insoluble tars precipitate out of the water with the water soluble organics are collected in the water as liquid smoke.
While there are hundreds of distinct chemical species present in liquid smoke, liquid smoke products have been characterized by three classes of chemicals according to distinct functional groups. The three classes are (1) acids, (2) carbonyls and (3) phenols
This functional definition is useful since phenols are the primary flavoring compounds while carbonyls are responsible mainly for coloration and acids serve as a preservative. Acids and carbonyls also make a secondary contribution to flavor and they enhance the surface characteristics of the meat products.
Acids are measured as titratable acidity calculated as acetic acid. Phenols are calculated as 2,6-dimethoxyphenol. The procedure for determining phenols is a modified Gibbs method. Carbonyls are calculated as 2-butanone. The procedure for determining carbonyls is a modified Lappan-Clark method. The procedures for determining carbonyls and phenols are described in U.S. Pat. No. 4,431,032 the contents of which are incorporated herein by reference.
A further measurement that is used to characterize liquid smoke is the browning index. The browning index is used in the smoke flavoring industry to measure the browning performance of a liquid. The browning index is a colormetric technique that measures the extent to which the carbonyls react with glycine. The browning index is determined from the difference between the adsorption at 400 nanometers of the glycine reacted solution and a control sample.
The application of liquid smoke solutions to meat and other food products can be carried out in a number of ways. Where the characteristic smoked color is desired, spraying or dipping can be done on individual items in a batch or continuous mode. Where large batches are to be processed an atomized cloud of liquid smoke can be employed. Alternatively, sausages and hams may be processed in casings into which liquid smoke solutions have been incorporated. In any case, where surface color is the primary effect which is sought, a measure of total carbonyls is used to judge the quantity of smoke required. These compounds react with the available amino groups of proteins at the surface to form the smoked color. The concentration of a specific carbonyl, hydroxyacetaldehyde, is also a good indicator of the color forming potential of liquid smoke.
Prior methods of producing liquid smoke suffer from relatively low yields of desirable products and relatively high yields of the undesirable by-products. In addition the levels of benzo[a]pyrene, a known carcinogen, is relatively high, requiring subsequent dilution of the collected condensable vapors with water to separate out these compounds.
The requirement to dilute the collected condensables to limit the level of benzo[a]pyrene below 0.5 ppb prevents the production of liquid smoke having a total acid content above 13% or a browning index above 13.0 without subsequent concentration.
Recently new methods have been developed for the rapid thermal processing of carbonaceous feedstocks. These methods have been called fast or flash pyrolysis.
Fast or flash pyrolysis of wood or cellulose is a method of imparting a high heating rate to the wood for a very short time and then rapidly quenching the pyrolysis products to a temperature below 350.degree. C. The heating rate for fast pyrolysis is greater than 1000.degree. C. per second and vapor residence times are below 2.0 second. While fast pyrolysis methods are known, the research and development in this area has concentrated on producing liquid and gaseous fuels, and on optimizing the production of high energy value fuels.
One object of this invention is to provide a method of using the water soluble products from fast pyrolysis to produce liquid smoke in place of conventional liquid smoke to achieve greater yields and higher concentrations of desirable product and lower yields of gaseous and solid by-products, resulting in greater efficiency and resulting cost savings.
Specifically fast or flash pyrolysis results in higher hydroxyacetaldehyde and other carbonyl yields and lower char, benzo[a]pyrene and gas yields. The higher carbonyl yields effects a higher browning index.
Further cost efficiencies results from a faster rate of the reaction in fast or flash pyrolysis which permits greater processing efficiencies in that smaller reactor volumes are required to process a given quantity of feedstock.
Another object of this invention is to provide a method for preparing a smoke colored and smoke flavored food product by treatment of the said food products with the aforementioned liquid smoke solution.
Other objects and advantages of the invention will become apparent from the ensuing disclosure and claims.